The present invention generally relates to structural fasteners and length measurements, and more particularly, to a grip length indicator integrated into a temporary clamping fastener, and to a method for determining a fastener grip length.
Structural fasteners come in varying length to accommodate any thickness of structures. The thickness of a structure may vary and, consequently, each hole where a fastener is to be installed needs to have its length measured in order to select and install the correct length fastener. Selecting the correct grip length of a fastener is critical in many applications, such as aircraft airframe installation. If a structural fastener is used that does not have the correct length, it may not be possible to install the fastener correctly and the joint strength may be reduced. To ensure the correct installation of fasteners, numerous types of measuring devices are available to measure the length of the hole where the fastener is to be installed. Typically, such measuring devices are gauges that include a measuring scale and that are formed like a fishhook. Such gauges can be hooked on the backside of a fastener hole, and the number at the scale that is even with the front side of the structure indicates the length of the hole. Usually, if the reading is between increments, the next higher increment is used for the selection of the fastener to be installed. Other measuring devices may require calibration. The gauging process of the fastener holes is typically done manually. In an installation process, for example, the installation of an aircraft airframe, the step of measuring the length of fastener holes is a very important step to ensure the correct selection of the fastener grip length and the correct installation of fasteners, but it is also a time consuming necessary step that slows the product flow. Furthermore, should a fastener be installed incorrectly because of use of an incorrect fastener length, the fastener would need to be removed and replaced, resulting in significant rework and potential damage to the structure.
Many structures, for example, in the aerospace industry or in the automobile industry, require temporary fasteners to be installed. Temporary fasteners hold the material in place until it is ready to be fastened permanently. Temporary fasteners are typically installed in structures that consist of a plurality of panels, such as a skin, a sealer or shim, and a substructure, for example, of an aircraft airframe. In an installation process, for example, of an aircraft airframe, temporary fasteners may be inserted into fastener holes in a first step. The temporary fasteners might need to be removed one by one and the length of each fastener hole might need to be measured in a subsequent step before the permanent fastener can be selected and installed. This process is time consuming and increases the cycle time.
Existing temporary structural fasteners include, for example, Wedgelock™ temporary fasteners manufactured by Monogram Aerospace Fasteners, Los Angeles, Calif., U.S.A. Wedgelock™ temporary fasteners are frequently used, for example, in the aircraft airframe installation process to temporarily hold a structure, such as substructure, sealant, and outer mold line skin, together. The Wedgelock™ temporary fastener is similar to the temporary fastener 10, as illustrated in FIG. 1. Temporary fastener 10 is a temporary blind fastener that has a cylindrical body 11. The cylindrical design eliminates fastener-to-tool orientation problems. The temporary fastener 10 further includes a captive nut 12, double lead thread (not shown), and larger geometry bearing wires 13. Utilization of a captive nut 12 provides a positive disengagement, allowing unobstructed removal of the fastener even if used in wet sealant applications. The double lead thread design (not shown) further enhances overall installation and removal efficiency while the large bearing area makes this type of fastener the optimum choice for both composite and metallic structures. The temporary fastener 10 is a power operated reusable temporarily fastener and may be installed and removed manually, automatically, or by a robot. Using the temporary fastener 10 may improve the installation and removal efficiency for temporarily fastening structures, but the temporary fastener 10 still needs to be removed from a structure to allow access to the fastener hole to be measured using a separate measuring device.
As can be seen, there is a need for a measuring device that enables accurate and efficient determination of a fastener hole length. Furthermore, there is a need to eliminate the step of gauging the fastener hole after the installation of temporary fasteners in order to reduce cycle time. Moreover there is a need to enable correct selection of the fastener grip length of the fastener to be installed.
There has, therefore, arisen a need to provide modifications to existing temporary fasteners that enable accurate determination of a fastener hole length without removal of the temporary fastener from the fastener hole. There has further arisen a need to eliminate the step of measuring fastener holes between the temporarily and the permanent installation of a fastener. There has still further arisen a need to provide a method for effectively determining of the correct fastener grip length for fastener holes of a large structure, such as an aircraft airframe.